Turning now to FIG. 1, a wireless communication system 10 is shown. The system 10 includes a source module 12 that provides digital data to an interface 14. The digital data can represent assorted types of information, such as audio, video, computer data and/or programs. The interface 14 buffers and/or streams the digital data to a media access controller (MAC) 16. The MAC 16 formats the digital data into a data stream. In some embodiments, the MAC 16 can also be configured to encode the data stream with a forward error correcting (FEC) code. The MAC 16 communicates the coded data stream to a physical layer (PHY) 18. The PHY 18 includes a plurality of transmit antennas 20-1, 20-2, . . . , 20-T, referred to collectively as transmit antennas 20. The PHY 18 modulates the coded data stream onto a plurality of OFDM subchannels that are transmitted by the transmit antennas 20.
The transmitted OFDM subchannels propagate through a communication channel 22. The communication channel 22 provides a plurality of paths, which are represented by arrows, between the transmit antennas 20 and receive antennas 24-1, 24-2, . . . , 24-R, referred to collectively as receive antennas 24. A signal propagation of each path can vary from the others in regard to attenuation, noise, multi-path fading, and other factors. These factors adversely affect a signal-to-noise ratio (SNR) in the subchannels as they are received at the receive antennas 24. The FEC facilitates decoding the received signal in the event that one or more paths reduce the SNR of one or more subchannels to the extent that they can no longer be successfully demodulated.
The receive antennas 24 communicate the received subchannels to a receive PHY 26. The receive PHY 26 demodulates the received subchannels into receive coded data stream and communicates it to a receive MAC 28. The receive MAC 28 extracts the digital data from the coded data stream and communicates it to a receive interface 30. The receive interface 30 buffers and streams the digital data to a destination module 32.
In some embodiments, the receive PHY 26 can use the spatial and/or SNR diversity of the receive antennas 24 and the communication paths, respectively, to compensate for, and/or minimize the effects of, the adverse factors in the communication channel 22. The PHY 18 can mitigate the adverse effect of multi-path fading by prepending a cyclic prefix to the data packet portion in each subchannel. The PHY 18 can also introduce diversity to the transmitted signals by adding a unique and constant cyclic delay to each subchannel.
Referring now to FIG. 2, a block diagram of a transmit PHY 50 is shown that uses such cyclic delay diversity (CDD). The PHY 50 receives a plurality of coded data streams 52-1, 52-2, . . . , 52-NS, referred to collectively as coded data streams 52.
Each of the coded data streams 52 is communicated to an input of a corresponding serial-to-parallel converter (S2P) module 54-1, 54-2, . . . , 54-NS, referred to collectively as S2P modules 54. Outputs of each S2P module 54 communicate with corresponding inputs of a mapping module 56. The mapping module 56 has a plurality of mapping module outputs 58-1, 58-2, . . . , 58-NT, referred to collectively as mapping module outputs 58. Each mapping module output 58 communicates with an input of a corresponding RF channel 60-1, 60-2, . . . , 60-NT, referred to collectively as RF channels 60.
The mapping module 56 maps the coded data streams to the mapping module outputs 58 according to a mapping matrix W[k], such as a Walsh or Fourier matrix. The variable k represents a subcarrier index of one RF channel 60 and k=0, 1, . . . , N−1, where N represents a total number of OFDM subcarriers. The mapping matrix W[k] can be chosen as
      W    ⁡          [      k      ]        =                    1                              N            T                              ⁡              [                  11          ⁢                                          ⁢          …          ⁢                                          ⁢          1                ]              T  when only one coded data stream 52 is being used. When a plurality of coded data streams 52 are being used, i.e., NS≧2 where NS represents the number of coded data streams 52, the mapping matrix W[k] can be chosen as the first NS columns of a unitary matrix of dimension NT×NT, such as a Hardamard matrix or a Fourier matrix, where NT represents the number of RF channels 60 being used.
Each mapping module output 58 communicates with an input of a corresponding inverse fast-fourier transform (IFFT) module 62-1, 62-1, . . . , 62-NT, referred to collectively as IFFT modules 62. The IFFT modules 62 transform the coded data streams from the frequency domain to the time domain. Each IFFT module 62 communicates its coded data stream to an input of a corresponding parallel-to-serial (P2S) converter 64-1, 64-1, . . . , 64-NT, referred to collectively as P2S converters 64. With the exception of the first RF channel 60-1, outputs of the P2S converters 64 communicate the coded data streams to the inputs of corresponding cyclic delay modules 66-2, . . . , 66-NT. Each cyclic delay module 66 adds the respective unique and constant cyclic delay to the corresponding coded data stream. The cyclic delays simulate spatial diversity between the transmit antennas by staggering modulation of the transformed data streams. The first RF channel 60-1 does not include a cyclic delay module 66 and effectively has a cyclic delay equal to zero.
Each of the cyclic delay modules 66-2, . . . , 66-NT communicates the corresponding coded data stream to a corresponding input of a cyclic prefix module 68-2, . . . , 68-NT. In the case of RF channel 60-1, the output of P2S converter 64-1 communicates the coded data stream to an input of a cyclic prefix module 68-1. The cyclic prefix modules 68-1, 68-2, . . . , 68-NT are referred to collectively as cyclic prefix modules 68. The cyclic prefix modules 68 prepend a cyclic prefix, which includes the last samples of the output of the corresponding IFFT module 62, to each corresponding coded data stream. A duration of the cyclic prefix is preferably greater than a predetermined difference in multi-path propagation arrival times at the receiver.
An output 70-1, 70-2, . . . , 70-NT of each of the cyclic prefix modules 68 communicates its coded data stream to respective D2A modules. An RF transmitter portion (not shown) of the PHY 50 then wirelessly transmits the coded data streams to the receiver. The transmit signals at the outputs 70 are respectively identified by the algebraic symbols x1, x2, . . . , xNt.
Referring now to FIG. 3, a block diagram of a transmit PHY 100 is shown. The PHY 100 uses a permuted space-frequency coding (PSFC) technique to achieve transmit diversity. The PHY 100 is similar to the PHY 50 with the exception of the mapping matrix W[k] and omission of the cyclic delay modules 66. With the cyclic delay modules 66 omitted, the outputs of the P2S converters 64 communicate directly with the inputs of corresponding cyclic prefix modules 68.
The PSFC technique chooses the mapping W[k] matrix as follows. Let Im be an identity matrix of dimension m×m and Im(k) be a matrix that is formed by circularly rotating the columns of Im to the left by k mod m. By way of non-limiting example, for m=3,
            I      3              (        0        )              =          [                                    1                                0                                0                                                0                                1                                0                                                0                                0                                1                              ]        ,            I      3              (        1        )              =          [                                    0                                0                                1                                                1                                0                                0                                                0                                1                                0                              ]        ,            I      3              (        2        )              =          [                                    0                                1                                0                                                0                                0                                1                                                1                                0                                0                              ]        ,            I      3              (        3        )              =          [                                    1                                0                                0                                                0                                1                                0                                                0                                0                                1                              ]        ,      etc    .  such thatI3(0)=I3(3)=I3(6)= . . . andI3(1)=I3(4)=I3(7)= . . . andI3(2)=I3(5)=I3(8)= . . .
The mapping matrix W[k] is formed by taking the first NS columns of
            1                        N          S                      ⁢          I              N        T                    (                  ⌊                      k            B                    ⌋                )              ,where B is a predetermined parameter selected to control the frequency selectivity of the corresponding RF channel 60.
It remains desirable in the art to provide alternative methods for providing transmit diversity such that OFDM receivers can minimize the effects the adverse factors have in the SNR of the received signals.